In recent years, as mobile devices improve in performance and acquire more functions, secondary batteries serving as their power supplies are required to have higher and higher capacity. Lithium-ion secondary batteries are drawing attention as secondary batteries that can satisfy this need.
In order to achieve a high capacity in a lithium-ion secondary battery, use of silicon (Si), germanium (Ge), tin (Sn), or the like as an electrode active material has been proposed. An electrode for a lithium-ion secondary battery (which hereinafter may also be simply referred to as an “electrode”) for which such an electrode active material is used is formed by applying on a current collector a slurry which contains an electrode active material, a binder, and the like, for example (called a “painted electrode”).
However, such electrode active materials undergo large changes in volume when occluding or releasing lithium ions, thus resulting in the problem of being pulverized through expansions and contractions that accompany repetitive charge and discharge. As an electrode active material becomes fine matter by being pulverized, a deterioration in the charge collecting ability of the electrode will occur. This will also increase the area of contact between the electrode active material and the electrolytic solution, thus promoting a decomposition reaction of the electrolytic solution by the electrode active material. This makes it impossible to obtain sufficient charge-discharge cycle characteristics.
Patent Document 1 and Patent Document 2 disclose, instead of a conventional painted electrode, forming an electrode active material layer on a current collector by using vapor-phase technique, liquid-phase technique, sintering technique, or the like. With an electrode which has been formed in this manner, as compared to a conventional painted electrode, the electrode active material's tendency to become fine matter through pulverization can be suppressed, and the adhesion between the current collector and the electrode active material layer can be enhanced, whereby deterioration in charge collecting ability can be prevented. Thus, it is expected that improvements in electrode capacity and cycle life beyond conventional levels are expected. Furthermore, whereas a conventional painted electrode would include an electrically conductive material, a binder, voids, etc., the methods of forming electrode active material layers disclosed in Patent Documents 1 and 2 can reduce or eliminate their amounts within the electrode, which permits an essential enhancement of the capacity of the electrode.
However, even with the aforementioned electrode, expansions and contractions of the electrode active material due to charge and discharge may cause problems such as peeling of the electrode active material layer from the current collector, wrinkles occurring on the current collector, etc., thus making it difficult to obtain sufficient cycle characteristics.
In view of this, it has been proposed to form an intermediate layer between the current collector and the electrode active material layer, thereby improving the adhesion between the current collector and the electrode active material layer and preventing peeling of the electrode active material layer from the current collector. Patent Document 3 discloses a construction in which a current collector composed of a metal or alloy having a high mechanical strength is used and an intermediate layer that can be alloyed with the electrode active material, e.g., copper (Cu), is provided between the current collector and the electrode active material layer. Patent Document 4 discloses a construction in which an intermediate layer containing molybdenum (Mo) or tungsten (W) is provided between the current collector and the electrode active material layer. Patent Document 5 discloses a construction in which an intermediate layer containing nickel (Ni) and titanium (Ti) is provided.
Patent Document 6 discloses performing an oxidation treatment for the surface of the current collector, and forming on the resultant oxide film an electrode active material layer which contains at least one of Si and Ge.
On the other hand, Patent Document 7 takes note of the fact that, in the case where silicon oxide is used as an electrode active material, the expansion coefficient of the electrode active material associated with charge and discharge varies depending on the oxygen content in the silicon oxide, thus proposing to increase the oxygen concentration in the electrode active material layer near the current collector so as to be higher than the average oxygen concentration in the electrode active material layer. In accordance with the construction of Patent Document 7, the expansion of the electrode active material layer near the current collector is reduced, thus suppressing deformation of the electrode due to expansion/contraction of the electrode active material layer.
[Patent Document 1] Japanese Laid-Open Patent Publication No. 11-339777
[Patent Document 2] Japanese Laid-Open Patent Publication No. 11-135115
[Patent Document 3] Japanese Laid-Open Patent Publication No. 2002-083594
[Patent Document 4] Japanese Laid-Open Patent Publication No. 2002-373644
[Patent Document 5] Japanese Laid-Open Patent Publication No. 2005-141991
[Patent Document 6] Japanese Laid-Open Patent Publication No. 2003-217576
[Patent Document 7] Japanese Laid-Open Patent Publication No. 2006-107912